448 ROSS G. HARRISON 
EFFECT OF REMOVAL OF MESODERM ALONE 
In order to remove the mesoderm alone, the ectoderm covering 
the limb region is first incised around three-fourths of its cireum- 
ference. It is carefully lifted from the underlying mesoderm 
and left hanging by its ventral border. The mesoderm is then 
removed from the region below the pronephros, all loose cells 
being cleaned off, as in experiments already described, and then 
the covering layer is finally stretched back into place and held 
for a short time by a glass bar. The ectoderm contracts con- 
siderably while the wound is being cleaned, but with the aid of a 
fine needle it can usually be drawn over the wound. In some 
cases perfect healing was obtained in less than an hour. In 
others small areas of yolk were found still uncovered on the day 
after the operation. The quickness of the healing seems, how- 
ever, to have no effect on the result, for in the three cases in 
which regeneration occurred, the healing was characterized as 
good, fair, and bad, respectively, while the cases of non-regen- 
eration followed both good and fair healing. 
Twelve experiments were made, in two of which the embryos 
died. In the ten remaining cases the wounds were of different 
sizes, varying from three to four segments in diameter, bounded 
as in the simple extirpations (p. 422). The results are given in 
table 8. 
In five cases the cleaned area was of the smallest size, ex- 
tending from the line between the second and third somites to 
that between the fifth and sixth. Three of these gave rise to 
regenerated limbs, while the other two did not. None of the 
other cases, which had larger wounds, regenerated.? In six of 
the cases which showed no regeneration the pronephros was left 
intact and in only one case was it removed. ‘These experiments 
differ from those described in the third section (p. 432) only with 
respect to the region from which the ectoderm covering the 
wound is taken. The results are in full agreement, and the 
corresponding figures given in table 4 and table 8 could with 
propriety be combined. 
13 In my preliminary note (Harrison, ’15), on the fifth line from the bottom 
of p. 542, the words ‘over three’ should be substituted for ‘four.’ 
